Many systems for production of three-dimensional modeling by photohardening have been proposed European Patent Application No. 250,121 filed by Scitex Corp. Ltd. on June 6, 1987, provides a good summary of documents pertinent to this art area, including various approaches attributed to Hull, Kodama, and Herbert. Additional background is described in U.S. Pat. No. 4,752,498 issued to Fudim on June 21, 1988.
These approaches relate to the formation of solid sectors of three-dimensional objects in steps by sequential irradiation of areas or volumes sought to be solidified. Various masking techniques are described as well as the use of direct laser writing, i.e., exposing a photohardenable polymer with a laser beam according to a desired pattern and building a three-dimensional model layer by layer.
However, all these approaches fail to identify practical ways of utilizing the advantages of vector scanning combined with means to maintain constant exposure and attain substantially constant final thickness of all hardened portions on each layer throughout the body of the rigid three dimensional object. Furthermore, they fail to recognize very important interrelations within specific ranges of operation, which govern the process and the apparatus parameters in order to render them practical and useful. Such ranges are those of constant exposure levels dependent on the photohardening response of the material, those of minimum distance traveled by the beam at maximum acceleration dependent on the resolution and depth of photohardening, as well as those of maximum beam intensity depend on the photospeed of the photohardenable composition.
The Scitex patent, for example, suggests the use of photomasks or raster scanning for achieving uniform exposure, but does not suggest a solution for keeping the exposure constant in the case of vector scanning. The use of photomasks renders such techniques excessively time consuming and expensive. Raster scanning is also undesirable compared to vector scanning for a number of reasons, including:
necessity to scan the whole field even if the object to be produced is only a very small part of the total volume, PA1 considerably increased amount of data to be stored in most cases, PA1 overall more difficult manipulation of the stored data, and PA1 the necessity to convert CAD-based vector data to raster data. PA1 excessive photohardening depthwise usually accompanied by inadequate photohardening widthwise. This problem becomes especially severe in cantilevered or other areas of the rigid object, which areas are not immediately over a substrate; PA1 loss in photospeed, due to local loss of the polymerization heat during photohardening; PA1 loss in resolution due to diffusion of heat away from the locus of photohardening; PA1 decreased shelf stability, due to differences in specific gravity between the radiation deflecting matter and the rest of the photohardenable composition. PA1 incorporating radiation deflecting matter in the photohardenable composition of highly different index of refraction in order to limit the depth of photohardening with simultaneous increase of the width of photohardening, so that the resolution is better balanced in all directions; PA1 using radiation deflection matter which has thermally insulating properties in order to improve the photospeed and resolution; PA1 using radiation deflecting matter which has comparable specific gravity as the rest of the photohardenable composition, in order to improve shelf stability. PA1 (a) forming a layer of a photohardenable liquid; PA1 (b) photohardening at least a portion of the layer of photohardenable liquid by exposure to actinic radiation; PA1 (c) introducing a new layer of photohardenable liquid onto the layer previously exposed to actinic radiation; PA1 (d) photohardening at least a portion of the new liquid layer by exposure to actinic radiation, with the requirement that the photohardenable composition comprises an ethylenically unsaturated monomer, a photoinitiator, and radiation deflecting matter, the deflecting matter being in the form of hollow spheres acting as a thermal insulator and having a first index of refraction, the rest of the composition having a second index of refraction, the absolute value of the difference between the first index of refraction and the second index of refraction being different than zero; and PA1 (e) successively repeating steps (c) and (d) until the three dimensional object is complete.
On the other hand, in the case of vector scanning only the areas corresponding to the shape of the rigid object have to be scanned, the amount of data to be stored is smaller the data can be manipulated more easily, and "more than 90% of the CAD based machines generate and utilize vector data" (Lasers & Optronics, January 1989, Vol. 8, No. 1, pg. 56). The main reason why laser vector scanning has not been utilized extensively so far is the fact that, despite its advantages, it introduces problems related to the inertia of the optical members, such as mirrors, of the available deflection systems for the currently most convenient actinic radiation sources, such as lasers. Since these systems are electromechanical in nature, there is a finite acceleration involved in reaching any beam velocity. This unavoidable nonuniformity in velocity results in unacceptable thickness variations. Especially in the case of portions of layers having no immediate previous levels of exposure at the high intensity it becomes necessary to use high beam velocities, and therefore, longer acceleration times, which in turn result in thickness non-uniformity. The use of low intensity lasers does not provide a good solution since it makes production of a solid object excessively time consuming. In addition, the usefulness of vector scanning is further minimized unless at least the aforementioned depth and exposure level relationships are observed as evidenced under the Detailed Description of this invention.
No special attention has been paid so far to the composition itself by related art in the field of solid imaging, except in very general terms.
Thus, the compositions usually employed, present a number of different problems. Such problems are:
Therefore, it is an object of this invention to resolve the problems cited above by:
European Patent Application 250,121 (Scitex Corp., Ltd.) discloses a three dimensional modelling apparatus using a solidifiable liquid which includes radiation transparent particles in order to reduce shrinkage.
U.S. Pat. No. 4,504,565 (Baldvins et al.) describes a radiation imageable composition in which an image can be produced upon exposure to intense radiation, the composition comprising (a) hollow ceramic microspheres, and (b) a binder material which will not be destroyed during exposure of the composition to intense radiation and will not mask the image produced upon exposure to intense radiation.